| This paper originates from two practical engineering problems.One is imaging behind screen.Screen pipe completion is often used in unconsolidated sandstone reservoirs.The function of screen pipe is to prevent formation sand from entering wellbore and to prevent formation sand from damaging pumping equipment.However,the screen pipe is easily blocked by sand production.Detecting the blockage of the screen pipe has become a practical issue of great concern to the production units,which is related to the formulation and implementation of the next production development plan.At present,there is no testing technology for sand accumulation outside the screen pipe.The second is the problem of pipeline defect detection.Nowadays,pipeline transportation is the main mode of transportation for oil and gas resources.Most of these transportation pipelines are buried in underground or shallow sea.Although there are protective measures such as paint coating,they will be defective due to long-term harsh environment.Defects in transportation pipelines can lead to leakage accidents,environmental pollution and property losses.Therefore,regular monitoring of the integrity of existing pipelines has become an urgent technical problem to be solved.In this paper,the finite difference numerical simulation method is used to study the feasibility of applying the ultrasonic leaky Lamb wave technology to the above two problems.The cylindrical layered screen pipe and pipeline model are approximated by a horizontal multilayer structure.The stress-velocity staggered grid finite-difference time-domain method(FDTD)in Cartesian coordinates is adopted and the mesh is divided into parallel computations by means of message-passing interface(MPI).A three-dimensional parallel finite-difference program for numerical simulation of leaky Lamb wave acoustic field in horizontal multilayer structure is developed independently.The parallel finite difference program is used to simulate the leaky Lamb wave logging in cased wells.By qualitatively and quantitatively analyzing of waveforms and snapshots,and comparing with the results of two-dimensional finite difference model in references,the correctness and accuracy of the finite difference program developed in this paper are verified.The first part of this paper is imaging behind screen.Leaky Lamb wave logging technology is a relatively mature technology for imaging behind casing.However,unlike in cased wells,when Lamb waves propagate in screen pipe,scattering and mode conversion will occur in holes,so it is necessaryto investigate whether Lamb wave logging technology can be applied to screen pipe.The leaky Lamb wave is excited by a pitch-catch mode in a 12 mm thick screen pipe with transmitter of 250 kHz at the center frequency.The effects of the width,length,interval of the screen hole(the distance between the screen holes in the same row)and the width of the sound channel(the distance between the two rows of screen holes)on the shape and attenuation rate of the received Lamb wave packet are investigated.The results show that the width of the sound channel has the greatest influence on the received waveform.The wider the sound channel is,the better to calculate the attenuation rate accurately.The other screen hole parameters have less influence.Then,the logging response of the rotary logging tool in the screen pipe are numerically simulated when the medium outside the screen pipe is water or sediment.For screen holes of 10 mm by 10 mm in regular arrangement,when the distance between the center of the transducers and the center of the screen holes is less than 5 mm,the attenuation rate can not be used to calculate the acoustic impedance of the medium outside the screen pipe.When the transducers and the screen holes overlap partially and the distance is greater than 5 mm,the attenuation can still be used to distinguish water and sediment outside the screen pipe even though the attenuation rate is less than that of the steel pipe.When the transducers is completely in the sound channel,the attenuation rate is close to that of the steel pipe,therefore can accurately calculate the acoustic impedance of the medium outside the screen pipe.It can be concluded that the attenuation rate of ultrasonic leaky Lamb wave can be used to calculate the acoustic impedance of the media outside the non-hole area of the screen pipe.Through the above numerical study show that it is feasible to use the ultrasonic leaky Lamb wave technology to detect the blockage outside the screen pipe.The second part of this paper is pipeline defect detection.In this paper,a water-coupled transducers of a transmitter,a transmission receiver and a reflection receiver is proposed.The center frequency of the transducer is 125 kHz.Leaky Lamb waves are excited and received in the pipelines and the received Leaky Lamb waves are used to detect pipeline defects.The transmission coefficients and reflection coefficients are defined as the amplitude of Lamb wave received by the transmitting and reflecting receivers when there is a defect,which are normalized by the amplitude of Lamb wave received by the transmitting receivers when there is no defect.Firstly,the influence of the inner and outer surface defects on the transmission and reflection coefficients of 12 mm thick pipelines with different widths and depths is investigated numerically.The reflection coefficient isinsensitive to the depth of inner and outer surface defects.The transmission coefficient decreases linearly with the increase of the depth of inner and outer surface defects,and the slope is-0.08/mm.The reflection coefficient and transmission coefficient are sensitive to the defect width.With the increase of the defect width,the reflection coefficient increases,and the slope is 0.08/mm.At the same time,the transmission coefficient decreases,and the slope is-0.09/mm.Finally,the numerical simulation of C-scan for a through-thickness defect with the size of 5mm * 8mm * 12 mm is carried out.As the distance between the defect and the transducers increases,the reflection coefficient decreases very slowly.Because of the interference between the scattering wave and direct Lamb wave,the transmission coefficient curve shows a 'W' trend and periodically fluctuation on both sides of the 'W' trend.The distance between the two minimum values in 'W' is exactly equal to the sum of two times the width of the transducer and the width of the defect..The defect width can be calculated accurately by using transmission coefficient.The defect depth can be further estimated by combining the reflection coefficient and transmission coefficient when the defect is col-linear with the transducers.Using the ratio of reflection coefficient to transmission coefficient can restrain the fluctuation outside the 'W' trend of transmission coefficient,which is beneficial to inversion when multiple defects exist.The numerical study in this paper proves the feasibility of the application of ultrasonic leaky Lamb wave technology in imaging behind screen and pipeline defect detection.Ultrasound Leaky Lamb wave imaging technology is proposed for the first time in this paper.These numerical results provide a theoretical basis for further instrument design and inversion imaging. |
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